1. Field of the Invention
This invention generally relates to electromagnetic vibration transducers and to methods of transducing vibrations, and in particular to such transducers having one element supported by springs for movement relative to another element thereof.
This invention has utility in any electromagnetic transducer that is motion sensitive, such as a seismometer, wherein relative movement between two members or elements induces a signal in a coil.
2. Description of the Prior Art
Seismometers which sense vibrations or tremors of the earth include two elements: one element coupled to the earth to move with it, and the other element to resist such movement due to its inertia. When conditions are ideal, the inertia element remains stationary while the element coupled to the earth moves with the earth. Under such ideal conditions, the relative movement between the elements is a faithful measure of the movement of the earth.
Attempts have been made to develop seismometers wherein the inertia element is the magnet and magnetic circuit, see for example U.S. Pat. Nos. 3,577,184 and 3,878,504. It has also been suggested to fix the coil to the magnet and to the pole pieces, as in U.S. Pat. Nos. 2,130,213 and 2,759,167. These seismometers, however, have not met with any commercial success, if they have ever been built.
In present day seismometers, the element that moves with the earth includes a magnet, and the inertia element (tending to remain stationary) includes a coil positioned in airgaps of the magnetic circuit. The relative motion between the coil and magnetic field induces a signal in the coil which is a measure of the earth's movement. These seismometers, generally known as moving coil seismometers, suffer however from well known drawbacks. Below will be listed only a few of such drawbacks to illustrate the problems which the novel variflux vibration transducers of this invention have solved.
1. If seismometers are poorly coupled to the ground, the signals which they produce are not truly proportional to the earth's movement. It is known that better results could be obtained from a seismometer having a relatively heavy inertia element. But in presently used seismometers, the weight of the mass-coil (coilform and coil thereon) is typically small in comparison to the total weight of the seismometer. For example, the inertia mass normally is only about 10% to 15% of the total mass of the seismometer. It has been a desirata, for a long time, to increase said mass ratio so that the seismometer would move in better unison with the earth, while the inertia mass remains stationary. One of the objects of this invention is to increase substantially said mass ratio.
2. The practice of interconnecting seismometers in groups to form arrays causes the seismometers to "communicate" with each other, which means that the mechanical excitation of one seismometer electrically energizes and moves the coils in all the other seismometers in the group. Obviously, if the excitation of one seismometer is due to noise, all the other seismometers in the group will also pick up the same noise. It is a further object of this invention to eliminate this communication problem between interconnected seismometers.
3. Most seismometers use either slip rings or pig tails, which are small, spirally wound wires, for providing electrical continuity to their moving coils. Both of these types of connections have well known disadvantages and advantages. It is a further object of this invention to preserve the advantages offered by the slip ring and the pig tail connections while eliminating their disadvantages.
4. Generally, present day seismometers attenuate, at about 12 db per octave, signals having frequencies below the natural frequency of the seismometer. It is an object of this invention to greatly increase this attenuation.
5. As seismic signals propogate through the earth, the earth attenuates the higher frequencies at a faster rate than it attenuates the lower frequencies. It is yet a further object of this invention to provide vibration transducers whose frequency response has a higher sensitivity to higher frequencies than to lower frequencies, in order to automatically compensate for the higher attenuation by the earth of the higher frequency signals.